1. Technical Field
The invention relates generally to semiconductor manufacture, and, in particular, to a method of fabricating large area, single crystalline diamond films.
2. Background Art
Semiconducting diamond exhibits a dielectric strength 50 times that of silicon and twice that of silicon carbide. It exhibits a saturated high field electron velocity 2.7 times that of silicon and GaAs. Its thermal conductivity is five times that of silicon carbide and twenty times that of silicon. Its dielectric constant is about half that of common semiconductors. For these reasons, diamond exhibits a figure-of-merit for high frequency power amplifiers that is 8200 times that of silicon and near eight times that of SiC. Its figure-of-merit for integrated circuits is 32 times that of silicon and twice that of SiC. Unfortunately, semiconducting diamond films have not been available in a size sufficient to use in the electronics industry as they have only been grown in suitable quality on natural diamond and natural diamond substrates are not large enough for electronic device processing purposes.
A few materials, such as copper, nickel, and boron nitride, have a lattice constant rendering them suitable for the heteroepitaxial growth of diamond. Most other materials have lattice constants too different from that of diamond so as to induce too many misfit dislocations in the diamond overgrowth and thereby render the diamond unsuitable for most electronic device applications. Thus, while single crystalline diamond films have been grown over large areas of silicon, they exhibit a rather severe lattice mismatch with silicon and the resultant films are characterized by many grain boundaries rendering them impractical and unsuitable for use in the manufacture of electronic devices.
Diamond has been shown to grow heteroepitaxially on single crystal nickel, but only with very sparse nucleation and more polycrystalline than crystalline character. Attempts to grow diamond on copper have also been without success.
In the epitaxial growth of all other semiconductors, the quality of the films grown increases as the growth temperature approaches the melting point of the material. The growth temperature must usually be at least 67% of the melting point of the material grown. Diamond films are typically grown at 900 Celsius whereas the melting temperature is about 3500 Celsius. The best homoepitaxial diamond films are grown at temperatures exceeding 1200 Celsius--but only when oxygen is present in the growth mixture and its concentration does not exceed that of the carbon present.
It is known that many metal carbides, e.g., niobium carbide, and nickel carbide, are converted to metal hydrides upon exposure to atomic hydrogen and that carbon is not soluble in the metal hydride.
It is well known that the melting point of very small particles is much less than that of the solid bulk melting temperature and that this particle melting point can typically be at a temperature of but 50% of the bulk counterpart melting temperature. For example, the variation of melting temperature with size of CdS nanocrystals is described in a report by A. N. Goldstein, C. M. Echer, and A. P. Alivisatos, "Melting in Semiconductor Nanocrystals", SCIENCE, Vol. 256, 5 Jun. 1992, pages 425-427.
Very thin heteroepitaxial films (e.g., three nanometers or less) also melt at temperatures lower that the bulk melting point. Related to this phenomenon is the melting of a thin meniscus on the surface of the bulk material at a temperature well below that of the bulk. This can be verified by impinging a low power laser on the surface and noting a large change in reflectivity as the surface begins to melt.
A procedure was perfected in the former USSR to manufacture nanocrystalline diamond powder in the late 1970s, but was not disclosed in any open publication. In 1992, they began to sell this powder to various groups and organizations throughout the world. Since then, the U.S. Navy Surface Weapons Center has learned how to manufacture diamond powder. More importantly, the Navy manufacturing procedure can reproducibly control the size of the diamond powder to crystallites at any given size between two and 1000 nanometers.
Scientists at the University of Alabama have shown the propensity of the Navy diamond powder to coalesce into polycrystalline diamond in the presence of hydrogen and heat. Stated differently, in the presence of hydrogen, it reorganizes itself into larger crystallites, but with not very long range order.
Scientists at Kobe Steel, USA and at North Carolina State University have demonstrated that they could place diamond grit (nominal 1 micrometer size particles) on a single crystal nickel surface, raise the temperature to 1200 Celsius in the presence of a plasma of atomic hydrogen and a mixture of simple hydrocarbon radicals while placing a d.c. bias on the substrate and nucleate sporadic spots of single crystalline diamond that is heteroepitaxially oriented with the substrate. Several papers have been published by scientists in the USA and Japan relating to the heteroepitaxial nucleation of diamond films on nickel. For example, two such papers by P. C. Yang, W. Zhu, and J. T. Glass, of North Carolina State University, have been published, one entitled "Nucleation of oriented diamond films on nickel substrates", J. Mater. Res., Vol. 8, August 1993, pages 1773-1776, and the other entitled "Oriented diamond films grown on nickel substrates", Applied Physics Letters, Vol. 63, No. 12, 20 Sep. 1993, pages 1640-1642. However, none have succeeded in anything but sporadic and nonuniform coverage of the nickel surface. Major portions of the surface are covered by unwanted polycrystalline diamond film.
In May, 1993, it was publicly disclosed that scientists at Sandia Laboratories had shown that the atoms on the surface of various metals and silicon could be moved on that surface in the presence of an electric field and at temperatures much lower that would otherwise be possible without the electric field. Presumably the same applies to the movement of carbon atoms on a surface.